Insider trading is the buying or selling of shares of a company by someone who has access to material, non-public information on the company.
Legal insider trading is when company insiders - officers, directors, employees and 10% owners - buy or sell shares in their company in accordance with securities laws and regulations. More ...

It's often said that insider selling can be a hint that they know that their stock is about to underperform the market. Even though this is true in some cases, it's also true that insider selling is frequently simply behind financial reasons of the insiders and have no connection with the company's prospects. More ...